Currently, as a refrigerant for refrigerators, car air conditioners, room air conditioners, industrial refrigerating machines or the like are widely used 1,1,1,2-tetrafluoroethane (R134a), which is one of hydrofluorocarbons (HFC), an R410A which is a mixed refrigerant of difluoromethane (R32) and pentafluoroethane (R125) in a mass ratio of 1/1 and the like. However, the use of these HFC refrigerants is regulated by the so-called F-gas Regulation, the purpose of which is to protect the global environment, because they have a high global warming potential (GWP) of 1000 or more although their ozone-depleting potential (ODP) is zero.
As a substitute candidate for a refrigerant with a high GWP, single 2,3,3,3-tetrafluoropropene (HFO-1234yf) or difluoromethane (R32) is studied because of the thermodynamic properties. In addition, these refrigerants and a mixed refrigerant of them with other refrigerants which have balanced properties with GWP are also studied. It is essential for the substitute for a HFC refrigerant to have a low GWP and HFO-1234yf has a low GWP of 4. R32 has a slightly high GWP of 675, however it is studied as a potential candidate because it has a high gas pressure and is a highly efficient refrigerant.
Further, hydrocarbon refrigerants such as isobutane (R600a) and propane (R290), which are practically used for refrigerators, have a low GWP of 20 or less and proper physical properties, and therefore are studied although their combustibility, and carbon dioxide (R744), which has a GWP of 1 (standard), is studied as a single refrigerant or a refrigerant to be mixed for the purpose of fireproofing.
When these refrigerants are used, the working fluid in which a refrigerant and a refrigerating machine oil are mixed together is required to have a higher wear resistance than that of the conventional one because of severe lubrication conditions.
In general, as a wear-resistant additive to improve the lubricity of a lubricating oil are known oily agents such as alcohols, esters and long-chain fatty acids, wear-resistant agents such as phosphates, metal dithiophosphates, and extreme-pressure agents such as organic sulfur compounds and organic halogen compounds. In the case of a refrigerating machine oil, alcohol-based or ester-based oily agents or triphenyl phosphate or tricresyl phosphate among phosphates are used because they do not precipitate even when coexisting with a refrigerant and only an additive which does not adversely affect stability can be used.
Furthermore, Patent Literature 1 suggests a lubricating oil including one for refrigerating machine in which a phosphorus-based additive and a specific epoxy compound are added in combination, Patent Literature 2 suggests a lubricating oil for a compressor in which a triphenyl phosphate and a tri(alkylphenyl)phosphate are added in combination for a HFC refrigerant, and Patent Literature 3 suggests a refrigerating machine oil in which a tricresyl phosphate and an epoxy consisting of a glycidyl ether or a carbodiimide are added for a HFC refrigerant.
However, oily agents among these additives form a lubricating film by adsorption, and therefore the friction coefficient can be maintained low in the case of relatively mild loading conditions such as a mixed lubricating area, however the antiwear effect is lost in the case of severe loading conditions. On the other hand, triphenyl phosphate and tricresyl phosphate have an insufficient wear resistance under severe lubrication conditions in which a refrigerant with a low GWP coexists although they have a certain degree of a wear resistance effect.
Because of these facts, a refrigerating machine oil with a higher wear resistance is demanded and a working fluid with a high wear resistance in which a refrigerant is mixed is required in a freezing/air conditioning system.